In a cellular environment, transmitted signals undergo multipath scattering. In particular, a signal transmitted by a base station/remote unit undergoes multiple reflections before being received at a receiver, and these reflections cause “echoes” (or copies) of the transmitted signal to be received by the receiver along with a non-reflected (or “prompt”) component. These echoes are generally of different amplitudes and different time delays, and therefore cause received signals to actually consist of a multiplicity of signals (the actual signal and its echoes), each having a different amplitude, angle of arrival, and time delay. In typical spread-spectrum communication systems a RAKE receiver is utilized to lock onto each of the several multipath rays of a received signal with individual RAKE “fingers.” The receiver then combines fingers together to form a demodulated information stream. Internal to the RAKE receiver a “searcher” (or search processor) will have knowledge of the time offsets and pilot power for each finger.
FIG. 1 shows a prior-art receiver 100 for receiving multipath signals. As shown, signal 102 enters RAKE fingers 101 and 103 of receiver 100 where Pseudo Noise (PN) despreaders 107 and 109 despread signal 102 to form multiple despread signals. As discussed above, signal 102 comprises a coded (spread) signal which has undergone multipath scattering over various communication paths. After despreading with the appropriate PN code, a chip stream results. The chip stream is further despread via Walsh despreaders 111 and 113 into a symbol stream representative of that channel data. After appropriate time correction (115, 117) and phase correction (119, 121), the resulting symbols are summed and passed to the remaining channel circuitry which consists of deinterleaver 125 and decoder 127. Although only one channel is shown, in actuality, a multiplicity of channels exist which requires, for each RAKE finger, a Walsh despreader, time correction and phase correction for each channel.
Because the symbol streams from each of the fingers are added together to form a combined symbol stream, the gains of diversity combining can be achieved for multipath signal reception. A problem exists, however, in that Code Division Multiple Access (CDMA) RAKE receivers must time and phase correct each channel's symbol stream for all fingers prior to combining. This requires complex hardware, the complexity of which being related to the number of channels currently being decoded by the receiver and the number of RAKE fingers being combined. Additionally, when multiple antennas are utilized to receive a scattered signal, the need to weight and combine multiple antennas along with independent PN despreading of data samples for each finger requires very high rate processing. Therefore, a need exists for a method and apparatus for combining multiple scattered signals that is less complex than prior art methods.